Chemical vapor deposition chamber

ABSTRACT

An improved deposition chamber deposits useful layers on substrates. The improved chamber includes a substrate edge protection system which, in combination with a purge gas, protects selected portions of the edge and underside of the substrate from the deposition gas while preventing the creation of a masked area on the substrate edge. The substrate is supported on a solid receiving plate, and a positioning assembly aligns the substrate to the receiving plate. In some embodiments, the invention may include a stem interconnected to the substrate, a heat limiting member disposed about the stem, and a shroud extending about the stem.

This divisional application of application Ser. No. 08/200,862, filedFeb. 23, 1994, now U.S. Pat. No. 5,695,568, which is acontinuation-in-part of Ser. No. 08/042,961, filed Apr. 5, 1993 nowabandoned.

BACKGROUND OF THE INVENTION

The present invention relates to methods and apparatus for depositinguseful layers of materials on substrates used in the manufacture ofsemiconductor die. More particularly, the invention relates to improvedapparatus and methods for use in the processing of substrates, such asby chemical vapor deposition.

Chemical vapor deposition, commonly referred to as "CVD," is used todeposit a thin layer of material on a semiconductor substrate. Toprocess substrates with the CVD process, a vacuum chamber is providedwith a susceptor configured to receive a substrate thereon. In a typicalprior art CVD chamber, the substrate is placed into and removed from thechamber by a robot blade. The chamber includes an intermediate substratepositioning assembly on which the substrate is located when it is placedinto, or about to be removed from, the chamber. To locate the substrateon the susceptor, the susceptor is passed through the center of thesubstrate positioning assembly to lift the substrate therefrom. Thesusceptor and the substrate are then heated to a temperature of between250°-650° C. Once the substrate is located on the susceptor and heatedto an appropriate temperature, a precursor gas is charged to the vacuumchamber through a gas manifold situated above the substrate. Theprecursor gas reacts with the heated substrate surface to deposit thethin material layer thereon. As the gas reacts to form the materiallayer, volatile byproduct gasses are formed, and these gasses are pumpedout of the vacuum chamber through a chamber exhaust system.

A primary goal of substrate processing is to obtain as many useful dieas possible from each substrate. Many factors influence the processingof substrates in the CVD chamber and affect the ultimate yield of diefrom each substrate processed therein. These factors include processingvariables, which affect the uniformity and thickness of the depositionmaterial layer deposited on the substrate, and contaminants that canattach to the substrate and contaminate one or more die therein. Both ofthese factors must be controlled in CVD and other processes to maximizethe die yield from each substrate.

One CVD processing variable which affects the uniformity of thedeposition material layer is the relative concentration of reacted andnon-reacted process gas components in the deposition chamber. Theexhaust system of the chamber includes a circumferential exhaust channellocated above the substrate adjacent the perimeter thereof, throughwhich the reacted process gas is vented. However, a gap exists in thecircumferential exhaust channel where the slit valve for moving thesubstrate into and out of the chamber passes through the chamber wall.The exhausting of reacted gaseous products from the chamber is lessefficient near this gap, and thus the exhausting of reaction products isnon-uniform in the chamber. This contributes to the creation ofnon-uniform deposition material layers on the substrate, because therelative concentration of reactive gas in the total gas volume in thechamber varies about the surface of the substrate due to the non-uniformexhausting of reacted gaseous products from the chamber.

In addition to the foregoing factor which affects the uniformity andthickness of the deposition material layer, CVD processing chambersinclude multiple sources of particle contaminants which, if received onthe substrate, reduce the die yield therefrom. One primary source ofparticulate contamination in CVD processing is the deposition materialdeposited on the chamber surfaces during processing. As the substrate isprocessed in the CVD chamber, a material layer is indiscriminatelydeposited on all surfaces within the chamber which are contacted withthe gas, such as the aforementioned lamp covers. If these chambersurfaces are later touched or rubbed, or if the material layer isloosely attached to the chamber surface and the chamber is shaken orvibrated, particles of deposition material layer can become free in thechamber and contaminate the substrate. Additionally, the depositionmaterial layer does not typically firmly attach itself to the edge andunderside of the substrate, and the layer formed in that location of thesubstrate is known to flake off the substrate and become a particlecontaminant.

One method of controlling particle generation in the chamber is to use ashadow ring to reduce the occurrence of the deposition layer on the edgeand underside of the substrate. A shadow ring is a masking member, whichis received on the susceptor and contacts the upper, outer,circumferential area of the substrate and limits access of thedeposition gas to the contacted area of the substrate. However, theshadow ring has several, limitations which contribute to the non-uniformprocessing of substrates. The volatile deposition gas still tends tomigrate under the lip of the shadow ring and deposit a material layer onthe substrate edge and underside which may later flake off.Additionally, the engagement of the shadow ring with the substrate cancreate particles. Finally, the shadow ring is a heat sink, which drawsheat out of the substrate and thus reduces the temperature of thesubstrate adjacent the area of contact between the substrate and shadowring, which affects the thickness of the deposition material layer onthe area of the substrate adjacent the shadow ring.

One alternative to the shadow ring is disclosed in European PatentApplication No. EPO 467 623 A3, published Jan. 22, 1992. In thatapplication, a shroud is provided around the perimeter of the substrate.The shroud includes a lip which overhangs, but does not touch, thesubstrate. A gas is provided to the underside of the substrate, and aportion of this gas flows outwardly between the substrate and susceptorand into a gap formed between the substrate and the shroud. Although theshroud creates a circumferential channel in which a non-reactive gas maybe maintained to mask the edge of the substrate, the structure shown inEPO 467 623 A3 has several disadvantages. First, when the shroud isreceived on the susceptor, it aligns with the susceptor and no means isdisclosed for aligning the substrate with the shroud. Any substantialmisalignment between the substrate and the susceptor will result insubstantial misalignment between the shroud and substrate, and theresulting annular gap between the substrate and shroud will benon-uniform around the perimeter of the substrate. This will createdifferential masking gas flow in different locations of the substrateedge. Second, introduction of the masking gas inwardly of the outerdiameter of the substrate can cause the substrate to float off thesusceptor during processing if the chamber pressure and process gas floware not closely controlled. Finally, the European application notes thatthe shroud disclosed therein masks the upper surface of the substrateand prevents depositions thereon, which reduces the useful area of thesubstrate.

A further source of substrate particulate contamination occurs where acracked, warped or substantially misaligned substrate is present in thechamber. If a cracked, warped or substantially misaligned substrate isencountered, substantial numbers of particulate contaminants can begenerated as the substrate is moved in the chamber. Additionally, iflarge segments of the substrate become free in the chamber, they mayseriously damage the chamber components. Finally, the upper surface andpassageways of the susceptor could be exposed to the corrosive reactivegas if a cracked, warped or misaligned substrate is processed in thechamber.

SUMMARY OF THE INVENTION

The present invention is useful as a CVD processing apparatus fordepositing blanket and selectively deposited tungsten, tungstensilicide, titanium nitride and other deposition materials with improveduniformity and controllability. The invention includes multipleembodiments which may be used independently or concurrently to improvethe control of processing variables and/or reduce the incidence ofcontamination of the: substrate during processing. Although theembodiments of the invention are discussed with reference to a CVDchamber, the embodiments of the invention are applicable to othersubstrate processes and process environments.

In a first embodiment of the invention, the apparatus includes a chamberhaving a substrate support member on which the substrate is located forprocessing. The substrate is positioned on the substrate support memberwith a positioning means, which positions the substrate above thesupport member for insertion and removal of the substrate through thechamber slit valve, and also positions the substrate on the surface ofthe support member for processing. The positioning means positions thesubstrate partially in conjunction with, and partially independently of,movement of the support member.

In a second embodiment of the invention, the substrate edge protectionsystem of the chamber includes a manifold extending circumferentiallyabout the edge of the substrate when the substrate is received on thesupport member. The manifold distributes a gas around the edge of thesubstrate. An alignment member may also be provided, to align thesubstrate edge with the channel. As the substrate does not touch theunderside of a shadow ring, particle generation is reduced, theuniformity of the deposition material layer is increased, and theuseable area of the substrate is increased.

To secure the substrate to the receiving plate during processing,depressions may be provided in the upper surface of the plate and portedto a vacuum source. In a further embodiment of the invention, a pressuresensor is disposed in the vacuum line which supplies the vacuum to thedepressions, which monitors pressure changes in the vacuum linecorresponding to the presence of a cracked, warped or substantiallymisaligned substrate on the support member. In an additional embodimentof the invention, a purge gas is circulated through the underside of thechamber, to protect the mechanical componentry of the chamber fromcorrosive attack and reduce the formation of deposits on the interiorchamber surfaces. In a still further embodiment of the invention, thechamber includes an extended pumping plate which extends around thecircumference of the chamber about the edge of the substrate and spansthe gap in the exhaust channel to uniformly pump reacted volatile gasfrom the chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

These, and other features and advantages of the invention will beapparent from the following description when read in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a partial sectional view of the processing chamber of thepresent invention, showing the receipt of a substrate on the substratesupport member thereof.

FIG. 2 is a sectional view of the processing chamber of the presentinvention positioned to receive a substrate therein for processing;

FIG. 3 is an additional sectional view of the processing chamber,positioned for processing a substrate therein;

FIG. 4 is a top view of the heater plate disposed in the chamber of FIG.2;

FIG. 5 is an additional top view of the heater plate of FIG. 5 having asubstrate thereon; and

FIG. 6 is a partial, sectional view of the heater plate and substrateedge protection system of FIG. 5 at 6--6.

BRIEF DESCRIPTION OF THE EMBODIMENTS Introduction

The processing chamber 10 of the present invention includes multiplefeatures and embodiments, which may be used independently, orconcurrently, to provide improvements in the structure and operation ofsubstrate processing chambers. Referring to FIG. 1, the cooperation andinteraction of several of these features are shown, including aninternally heated substrate support member, or heater plate 18, asubstrate edge protection system 30 in the form of a purge gas channel220, a substrate alignment member 32 in the form of a plurality ofalignment pins 224 on the upper surface of the heater plate 18, and animproved chamber exhaust system 300.

The heater plate 18 is actuable upwardly within chamber 10 to receive asubstrate 24 thereon for processing, and downwardly in chamber 10 toposition the substrate 24 therefrom for removal from chamber 10. Toposition the substrate 24 on the heater plate 18, a plurality of supportpins 25 are provided. These support pins 25 pass through the body of theheater plate 18 and may be extended from the heater plate 18 to receivethe substrate 24 as it is placed in the chamber 10 by the robot blade.The heater plate 18 may move upwardly with respect to the support pins25 to position the substrate 24 on the heater plate 18 for processing,and downwardly with respect to the support pins 25 to position thesubstrate 24 above the heater plate 18 for removal of the substrate 24from the chamber 10 by the robot blade. For ease of illustration,support pins 25 are shown in FIG. 1 as sinking into heater plate 18.However, it will be appreciated that as the substrate 24 is beingreceived on heater plate 18, support pins 25 and substrate 24 arestationary and heater plate 18 is actually moving upwardly.

To reduce the occurrence of the deposition layer on the substrateunderside and edge, heater plate 18 includes an edge protection system30, preferably in the form of an integral, circumferential, purge gaschannel 220 positioned adjacent the edge 27 of a substrate 24 when thesubstrate 24 is received on the heater plate 18. Once the substrate 24is located on heater plate 18 and processing begins, a continuous flowof purge gas is provided to channel 220 about the edge 27 of thesubstrate 24 to ensure that little, or no, deposition will occur onundesirable portions of the edge 27 of the substrate 24, or on theunderside of the substrate 24 immediately adjacent the edge thereof. Tofully exploit the purge gas channel 220, the positioning of thesubstrate 24 on the heater plate 18 is critical, as any grossmisalignment will place a portion of the substrate 24 edge in a positionwhich substantially blocks the channel 220. Therefore, the heater plate18 includes a substrate alignment member 32, which includes a pluralityof tapered guide pins 224 provided above channel 220 along the perimeterthereof to guide the substrate 24 onto the heater plate 18. An eccentricand/or misaligned substrate 24 will engage one or more guide pins 224 asthe substrate 24 is received onto the heater plate 18. As the heaterplate 18 approaches the substrate 24 supported over the heater plate 18on support pins 25, the substrate 24 will be forced toward the center ofthe heater plate 18 at the portion of the substrate 24 edge 27 whichtouches a guide pin 224. This aligns the entire circumference of thesubstrate 24 in the proper position with respect to the purge gaschannel 220 to ensure passage of purge gas over the entire edge 27 ofthe substrate 24, except in the very small area where the edge 27contacts a pin 224.

During processing, the substrate 24 is commonly maintained at anelevated temperature. To establish and maintain this temperature, theheater plate 18 of the present invention includes a resistive heatingelement therein. The heater plate 18 heats the substrate 24, whichcommonly enters the chamber at a temperature lower than that of heaterplate 18. As the substrate is heated to the processing temperature, thesubstrate edge 27 may load against one or more of the guide pins 224,and if significant thermal expansion occurs thereafter, the substrateedge 27 may chip. To address this problem, chamber pressure may bemaintained in the plurality of vacuum grooves 77, 78, which are providedin the heater plate upper surface 26 to chuck the substrate 24 to heaterplate 18 during processing. Alternatively, gas may be introduced thereinto reduce the fictional adhesion of the substrate 24 to the heater plateupper surface 26 and thus allow the substrate 24 to expand away fromguide pins 224 as the substrate 24 thermally expands.

To increase the uniformity of the exhaust of reacted gaseous productswithin the chamber 10, the exhaust manifold 23 of the chamber receives apumping plate 308 thereover, which includes a series of spaced apertures29 therein. The apertures 29 are evenly spaced about the entirecircumference of the manifold 23, and the plate 308 spans the gap in themanifold 23 created by the presence of the slit valve 11 in the chamberwall, to increase the uniformity of the removal of reacted gaseousproducts from the chamber.

These features, and other additional features of the invention describedherein, may be used independently or concurrently to provideimprovements in the processing of substrates in the chamber.

The Processing Chamber

Referring now to FIGS. 2 and 3, the multiple improvements and featuresof the chamber 10 of the present invention are shown in a CVD processingapparatus. In FIGS. 2 and 3, the chamber 10 is shown in partial cutawayto show the interaction and interconnection of the improvements andfeatures of the chamber 10. In FIG. 2, the chamber 10 is shown with theheater plate 18 in the retracted position, wherein a substrate 24 may bepositioned on, or removed from, the heads of the support pins 25 whichextend from the upper surface of the heater plate 18. In FIG. 3, theapparatus is shown with the heater plate 18 in the extended positionwith the support pins 25 sunk into the heater plate 18 to position thesubstrate 24 on the heater plate 18 for processing. Although thefeatures and improvements of the chamber 10 are shown in FIGS. 2 or 3,the discussion of these features may include other Figures necessary toshow the details of the features and improvements.

The CVD processing apparatus of FIGS. 2 and 3 generally includes achamber 10 having an outer wall 12, a cover 14 and a base 16 which forman evacuatable enclosure 13 in which the vertically-moveable substratereceiving heater plate 18 is located. Heater plate 18 is moveable withinenclosure 13 to position a substrate 24 thereon for processing. Heaterplate 18 preferably includes the substrate edge protection system 30 asan integral part thereof.

The Heater Plate and Stem Assembly

The heater plate 18 is movable vertically in the enclosure 13 by thestem 20, which is connected to the underside of heater plate 18 andextends outwardly through the base 16 of the enclosure 13 where it isconnected to a drive system 22. The stem 20 is preferably a rightcircular, tubular, aluminum member, having an upper end 40 disposed insupporting contact with the underside of the heater plate 18 and a lowerend 42 closed off with a cover plate 43. The stem lower end 42 isreceived in a cup shaped sleeve 96, which forms the connection of thestem 20 to the drive system 22. To provide connections from the exteriorof the chamber into the heater plate 18, cover plate 43 and sleeve 96include a plurality of aligned apertures therein, through which theheater plate connections are maintained. Stem 20 mechanically positionsthe heater plate 18 within the enclosure 13 and also forms an ambientpassageway through which a plurality of heater plate connections extend.

The heater plate 18 is configured to provide heat to a substrate 24received on the upper surface 26 thereof, while minimizing the heattransfer therefrom along the stem 20. The heater plate 18 is preferablya solid aluminum member, which is welded to the upper end 40 of stem 20.Preferably, the heater plate is heated by a resistive heating elementlocated therein, to provide sufficient heat to maintain the uppersurface 26 of the heater plate 18 at elevated processing temperatures ofbetween 250 and 650 degrees celsius. To power the heating element, theelement preferably includes a downwardly projecting tubular portion,which terminates in a blade type connector 64 in the cover plate 43. Amating blade connector 62 is located in the sleeve 96, to mate with, andprovide electric power to, the connector 64 in the cover plate 43.

The Heater Plate Thermocouple Connection

Referring to FIG. 2, a thermocouple 56 is provided in heater plate 18 tomonitor the temperature thereof. The heater plate 18 includes a bore 50,extending upwardly therein and terminating adjacent, but interiorly of,the heater plate upper surface 26. This bore 50 provides a pilot toreceive the end of the thermocouple 56 therein, and also provides anaperture for the receipt of the purge gas and vacuum supplies into theheater plate 18. The bore is preferably formed by boring a through holein the heater plate upper surface 26, and extending a plug 51 and aconnector housing 53 into the bore. The upper surface of the bore 51 maybe slightly recessed from the upper surface 26 of the heater plate 18,or may be ground or otherwise configured to provide a continuous heaterplate upper surface 26. The connector housing 53 and the plug 51 may beformed as separate elements, or as one continuous element. Thethermocouple 56 is configured as a rigid rod, which extends through apair of aligned apertures in the cover plate 43 and the sleeve 96, andterminates within the bore 50 in contact with the solid mass of theheater plate 18 and/or the connector housing 53. The lower end of therigid rod includes a bracket 59, which is releasably attached to theexterior of the sleeve 96, to maintain the thermocouple 56 in positionin the heater plate bore 50. Preferably, the bracket is maintained onthe exterior of the sleeve 96 by a plurality of screws, but otherattachment means, such as clamps or spring clips, may be substituted forthe screws. The thermocouple 56 is connected to an amplifier and afilter for temperature display and over-temperature protection. Toensure that air is present in the bore 50, the rigid rod may be slightlysmaller in diameter than the aligned apertures in the cover plate 43 andthe sleeve 96. Thus, ambient air will pass through the aligned aperturesabout the thermocouple 56 and thus be present about thermocouple 56within the bore 50 in the heater plate 18 to increase the heat transferbetween the mass of the heater plate 18 and the thermocouple 56 toincrease the accuracy and response time of the thermocouple.

The Purge and Vacuum Supplies

Referring now to FIG. 3, the purge gas supply for supplying a protectivegas to the substrate edge protection system 30 is shown. Purge gas pipe52 extends through stem 20, from cover plate 43 to the connector housing53 in the heater plate 18. The connector housing 53 includes a pluralityof bores therein, which register with purge gas and vacuum bores in theheater plate 18. A purge gas bore 70 extends within the heater plate 18,and into a corresponding bore in the connector housing 53, to supplypurge gas from the connector housing 53 to the upper surface 26 of theheater plate 18. In the preferred substrate edge protection system, thebore 70 supplies purge gas to a manifold 218 which is ported to aplurality of purge gas apertures 234 extending through the upper surfaceof heater plate 18 into channel 220 as shown in FIG. 6.

Referring now to FIGS. 3 and 4, the vacuum supply to heater pipe 18 isshown. Vacuum pipe 48 passes through stem 20 from the cover plate 43 onthe lower distal end 42 of the stem to upper end 40 of the stem, and isconnected through the connector housing 53 in the heater plate 18 to aplurality of vacuum ports 76 extending into a plurality of individualvacuum grooves 77, 78 in the upper surface 26 of heater plate 18. Tosupply the vacuum ports 76, a plurality of cross bores 75 are drilledinto the heater plate 18 immediately below the upper surface 26, andthese cross bores 75 all align into a corresponding bore in theconnector housing 53. The vacuum pipe 48 terminates into thecorresponding bore in the connector housing 53, and thus a vacuum iscommunicated through the vacuum pipe 48 and into the grooves 77, 78.

The cover plate 43 and the sleeve 96 include aligned apertures to supplythe purge gas and vacuum supplies into the purge gas pipe 52 and vacuumpipe 48 in stem 20, in addition to the apertures therethrough throughwhich the thermocouple 56 and heater element connections extend. Thepurge gas supply, and the vacuum, are preferably supplied to the sleeve96 through bellows tubing, which is connected into fittings threadedinto the proper apertures in sleeve 96. To prevent leakage of the vacuumor purge gas at the interface of the cover plate 43 and the sleeve 96, acircumferential groove is provided about the interface of the alignedapertures through which the vacuum and purge gas supplies aremaintained. The grooves are preferably located about the exit of theapertures from the upper end of the sleeve 96, and o-ring seals arelocated in the grooves to seal any gap between the cover plate 43 andthe sleeve 96 at the aligned apertures. The use of wrings to seal thegas and vacuum apertures, in conjunction with the use of a bladeconnector 64 to connect the heating element to the power supply and theuse of a rigid rod as the thermocouple, permits relatively simpledisassembly of the sleeve 96 from the stem 20.

The Heater Plate Positioning Assembly

The heater plate positioning assembly 34 for positioning the heaterplate at multiple locations within the chamber enclosure includes thestem 20 interconnected to the drive system 22. Stem 20 is connected tothe underside of the heater plate 18, and extends outwardly of the base16 to connect to the drive system 22. The drive system 22 includes amotor and reduction gearing assembly 82 suspended below enclosure 13 andconnected with a drive belt 84 to a conformable coupling and lead screwassembly 86. A transfer housing 88 is received on lead screw assembly86, which is guided up and down and held against rotation by a linearslide 90. Transfer housing 88 extends about the circumference of stem20, and is attached to the lower distal end 42 thereof through the endsleeve 96, to move and support stem 20 and heater plate 18 thereon. Themotor actuates the lead screw assembly 86 to move stem 20 and heaterplate 18 thereon. A seal ring 126 is provided in a groove in stem 20 toseal the outer surface of the lower end 42 of stem 20 in sleeve 96.

Heater plate 18 can droop or sag along its outer edge at the hightemperatures used in CVD processing. To reduce the likelihood ofmechanical deformation at the high CVD process temperature, supportsleeve 81 is provided to radially support heater plate 18. Sleeve 81includes a lower tubular portion 83, preferably formed of aluminum,received on a ledge 85 on stem 20. The ledge 85 may be formed, forexample, by locating a snap ring in a groove in the stem 20, whichprojects radially from stem 20 adjacent stem lower distal end 42 or bymachining a circumferential boss on,stem 20. A spring 87 is received onledge 85 to receive the base of lower tubular portion 83 thereon toupwardly bias sleeve 81. The upper end of sleeve 81 terminates in anoutwardly radiating support flange 89, on which a support ring 91,preferably a ceramic ring with high resistance to sagging at elevatedtemperatures, is received. The flange 89 includes an inner circularalignment boss 93 and an outer, upwardly extending lip 95. The boss 93extends into the central aperture in ring 91 to align the ring 91 on theboss 93. The support ring 91 is supported on the lip portion 95, tominimize the contact area between the support ring 91 and the sleeve 81.Additionally, a plurality of apertures extend through lip 95, whichallow gasses trapped in the interior of the sleeve 81 to vent outwardlytherethrough, along the underside of the support ring 91. Support ring91 presses against the lower ring 21 of heater plate 18, and ismaintained in contact therewith by the upward bias of spring 87. Theceramic does not lose strength at the elevated processing temperature,and thus the ring 91 supports the heater plate 18 against sagging. Toprotect the stem 20 and maintain a vacuum thereabout, a shroud 94extends downwardly about stem 20 from the underside of chamber base 16and terminates on lower end sleeve 96. Shroud 94 and the outer surfaceof stem 20 extending below aperture 100 form annulus 127 therebetween.The annulus 127 communicates with the interior of enclosure 13 throughaperture 100, and is thus maintained at the same vacuum pressure asenclosure 13. Shroud 94 includes a pair of bellows 98, 99 and a transferring 102 which seals the area about the outer circumferential surface ofthe stem 20 from the atmosphere. Each bellows 98, 99, terminates in asupport ring 106 a-d. Each support ring 106 a-d is a generally rightcircular member which includes an outwardly projecting support portion112. On support rings 106 a-c, a seal ring is disposed in projectingsupport portion 112 to seal the annulus 127 at the support rings 106a-c. The lower end of annulus 127 is sealed by the interconnection ofsleeve 96 and transfer housing 88. Seal ring 126, disposed in stemdistal end 42, seals the base of stem 20 to sleeve 96 and thus completesthe sealing of annulus 127 from the atmosphere.

When a substrate is processed in chamber 10, the volatile reaction gaswill migrate to the bottom of the enclosure 13, and then down throughthe aperture 100 and into contact with bellows 98, 99, transfer ring 102and support rings 106 a-d. The heat generated by the electric resistanceheating element to heat the heater plate 18 for substrate processingconducts through stem 20 to heat bellows 98, 99, support rings 106 a-d,drive system 22 and transfer ring 102. The heat radiated and conductedby the stem 20, in conjunction with the presence of the reactive gas,creates a corrosive environment for support rings 106 a-d, transfer ring102 and bellows 98, 99.

Chamber Component Protection System

To reduce the heating of the stem 20 by heating element within heaterplate 18, the stem 20 is manufactured from one material, preferably analuminum alloy such as 5086 or 5083 aluminum, and the heater plate 18 ismanufactured from a pure aluminum, preferably an 1100 aluminum, or otheraluminum material having at least 99% A1 and less than 0.05% Mg. The1100 aluminum material may be used in the CVD environment, and need notbe anodized. The aluminum material of stem 20 preferably has a smallercoefficient of thermal conductivity than the heater plate 18, and thuswill transmit heat from heater plate 18 less efficiently than would astem of a pure aluminum. Additionally, a heat choke portion 44, with areduced cross-section and preferably 4" in length, is provided on stem20 adjacent heater plate 18, through which a sufficient temperaturegradient may be established between heater plate 18 and the lower distalend of stem 20 so that a low-cost flouroelastomer material such as aViton® material may be used in seal 126.

To reduce the temperature of the components below the enclosure 13 whichare otherwise raised by the heat which is transferred down stem 20 pastheat choke portions 44, and to quickly reduce the temperature of theentire assembly when servicing is necessary, water may be provided tocoolant passages provided within sleeve 96. Alternatively, a waterjacket may be placed around the sleeve 96, or transfer case 88 andtransfer ring 102, to help cool these components during and aftersubstrate 24 processing. Further, a cooling fan may be used to pass airover the components to increase heat transfer therefrom.

To limit the introduction of reactive gas into annulus 127 about stem 20and shroud 94, sleeve 96 also includes a purge gas manifold 97 formed atthe interface of-the sleeve 96 and the lower support ring 106d, intowhich a supply of purge gas, such as Argon, may be provided. The purgegas flows outwardly from the manifold 97, from a plurality of holesspaced about the manifold 97, preferably 8 to 12 holes, and thenupwardly through annulus 127, to maintain a gas barrier against entry ofreactive gasses through aperture 100 into annulus 127. The flow of purgegas through the manifold 97 is preferably maintained at a flow ratewhich will maintain laminar plug flow of purge gas upwardly in theannulus 127. By maintaining these conditions, the diffusion of thereactive gas downwardly through the aperture 100 will be substantiallyeliminated. Additionally, during processing, the purge gas passes upthrough aperture 100 and about the outer edge of the heater plate 18 tominimize the passage of reactive gas down about the sides of heaterplate 18. This reduces the amount of reactive gas which reaches theinterior surfaces of the structural components of the chamber, and theunderside of the heater plate 18, to reduce the amount of unwantedmaterial deposition which may occur on these surfaces.

The Substrate Positioning Assembly

Stem 20 moves upwardly and downwardly through the aperture 100 in base16 of enclosure 13, to move heater plate 18 to receive a substrate 24thereon and, after processing, to move heater plate 18 into a positionwhere the substrate 24 may be removed from the enclosure 13 by a robotblade. To selectively support the substrate 24 in a position above theheater plate 18, the substrate positioning assembly 140 includes aplurality of support pins 25 which move with respect to heater plate 18to support the substrate 24 in position to be placed in, or removedfrom, the enclosure 13, and to locate the substrate 24 on the heaterplate 18. The support pins 25 are received in sleeves in bores 130disposed vertically through heater plate 18. Each pin 25 includes acylindrical shaft 132 terminating in a lower spherical portion 134 andan upper truncated conical head 136 formed as an outward extension ofshaft 132. Bore 130 includes an upper, countersunk portion 138 sized toreceive enlarged head 136 therein such that when pin 25 is fullyreceived into heater plate 18, head 136 does not extend above thesurface of heater plate 18.

Referring now to FIGS. 2 and 3, support pins 25 move partially inconjunction with, and partially independently of, heater plate 18 asheater plate 18 moves within enclosure 13. Support pins 25 must extendfrom heater plate 18 to allow the robot blade to remove the substrate 24from the enclosure 13, but must also sink into heater plate 18 to locatethe substrate 24 on the upper surface 26 of heater plate 18 forprocessing. To provide this positioning, a substrate positioningassembly 140 is provided which is normally biased upwardly intoenclosure 13, but is also moveable downwardly by the stem 20 as stem 20moves to actuate heater plate 18 downwardly in enclosure 13.

Substrate positioning assembly 140 includes an annular pin support 142which is configured to engage lower spherical portions 134 of supportpins 25, and a drive member 144 which positions pin support 142 toselectively engage support pins 25 depending upon the position of heaterplate 18 within chamber. Pin support 142 includes an upper pin supportring 146, preferably made from ceramic, which extends around theunderside of heater plate 18 to selectively engage the lower sphericalportions 134 of support pins 25, and a sleeve portion 150 which extendsdownwardly from pin support ring 146 through aperture 100 to terminateon transfer ring 102. Transfer ring 102 is disposed circumferentiallyabout stem 20, and is keyed to slide 90 to prevent rotation thereof.

The sleeve portion 150 includes a lower cylindrical portion 149, and anoutwardly extending radial support 151 which receives and supports pinsupport 146 thereon. The radial support 151 includes an upper, generallyflat, upper surface having a circumferential alignment wall 153 whichaligns with the inner diameter of the annular pin support 146 and aplurality of upwardly supporting support ribs 155 which support theunderside of the pin support ring 146. During the operation of thechamber 10, gases may become trapped along the interior of cylindricalportion 149, which could damage the chamber components. To relieve thesegases, a plurality of gaps 157 are formed adjacent the support ribs 155,and a plurality of holes 159 are formed through lower cylindricalportion 149. The holes 159 and gaps 157 allow the free flow of gasesfrom the interior to the exterior of the sleeve 150.

Pin drive member 144 is located on the underside of enclosure 13 tocontrol the movement of sleeve portion 150 with respect to heater plate18, and includes therefore a spring assembly 156 which is connected totransfer ring 102 to provide the upward bias on transfer ring 102 andsleeve portion 150 to bias pin support ring 146 upwardly to push thesupport pins 25 upwardly through the heater plate 18, and the snap ringor ledge 84 on stem 20 which is selectively engageable with sleeve 150to move sleeve portion 150 and pin support ring 146 attached theretodownwardly after heater plate 18 has moved a preselected distancedownwardly in enclosure 13. Spring assembly 156 includes a housing 158having a slot 160 therethrough, which is attached to the underside ofenclosure 13 adjacent aperture 100. A spring-loaded finger 154 extendsthrough slot 160, and is upwardly biased by a spring 164 in housing 158.Finger 154 is rigidly connected to transfer ring 102, and thus upwardlybiases sleeve 150 attached thereto. The upper terminus of housing 158limits the upward movement of finger 154. Transfer ring 102 is alsorigidly connected to support ring 106c, which includes a downwardlyextending tubular portion which terminates in an inwardly extendingflange 173. The ledge 85, which supports sleeve 81 on stem 20, is alsoengageable against flange 173 as stem 20 moves downwardly.

When heater plate 18 is fully upwardly extended in enclosure 13 forprocessing, finger 154 is fully actuated against the upper end ofhousing 158, and pin support ring 146 is disposed below heater plate 18such that the lower spherical portions 134 of support pins 25 are spacedtherefrom. When processing is completed, stem 20 moves downwardly tomove heater plate 18 downwardly in enclosure 13. As this movementcontinues, lower spherical portions 134 of pins engage pin support ring146. At this point, finger 154 is biased against the top of housing 158and both the finger 154 and the pin support ring 146 coupled theretoremain stationary. Thus, once lower spherical portions 134 engage pinsupport ring 146, support pins 25 remain stationary and support thesubstrate 24 in a stationary position within chamber as heater plate 18continues moving downwardly. After a preselected amount of heater plate18 travel, the ledge 85 on stem 20 engages flange 173, which locks stem20 to sleeve 150 and causes heater plate 18 and pin support ring 146 tomove downwardly in unison. Once the ledge 85 engages flange 173, thesupport pins 25 remain stationary with respect to heater plate 18, andboth elements move downwardly in enclosure 13. Once heater plate 18 andsubstrate 24 suspended thereover on support pins 25 are in the properposition, the robot blade enters through the slit valve 11, removes thesubstrate 24, and places a new substrate 24 on support pins 25. Stem 20then moves to move sleeve 150 and heater plate 18 upwardly. When finger154 engages the top of housing 158 sleeve 150 becomes stationary, whileledge 85 moves off flange 173 as stem 20 continues moving upwardly, andthus continued movement of heater plate 18 sinks support pins therein toposition the substrate 24 thereon for processing. By moving the supportpins 25 partially in conjunction with, and partially independently of,the heater plate 18, the overall length of the support pins 25 may beminimized, and the length of the pin shaft 132 which is exposed belowthe heater plate 18 and support ring 91 during processing is equal tothe distance from the heater plate 18 that the substrate 24 is locatedwhen the substrate 24 is being manipulated on and off the support pins25 by the robot blade. Thus, minimal support pin 25 surface area isexposed during processing, and therefore minimal deposits will occur onthe support pins 25.

The Vacuum Clamping System

Referring now to FIGS. 3 and 4, the vacuum clamping mechanism of thepresent invention is shown. The upper surface 26 of heater plate 18includes a plurality of concentric grooves 78 therein, which intersectwith a plurality of radial grooves 77. A plurality of vacuum ports 76,preferably three per radial groove 77, are disposed to communicatebetween the base of each radial groove 77 and the circular vacuummanifold 75 disposed within heater plate 18. Vacuum pipe 48 communicateswith manifold 75 to supply the vacuum thereto.

Vacuum ports 76 and grooves 77, 78 provide a low pressure environmentunder the substrate 24 to chuck the substrate 24 to the heater plateupper portion 26. During processing, enclosure 13 may be maintained atabout 80 torr. To adhere the substrate to heater plate top surface 26during processing, a vacuum of 1.5 torr to 60 torr is drawn throughvacuum pipe 48, and thus through ports 76 to grooves 77, 78. The 20 to78 torr pressure differential between grooves 77, 78 and enclosure 13causes the substrate 24 to adhere to the heater plate top surface 26 toincrease the heat transfer from the heater plate 18 to the substrate 24.After processing, grooves 77, 78 may maintain a lower pressure than thatpresent in enclosure 13, which may firmly adhere the substrate 24 toheater plate upper surface 26. In that instance, support pins 25 cancrack the substrate as it is forced off the heater plate 18. To equalizethe pressure existing in grooves 77, 78 with that existing in enclosure13, a bypass line may be provided between the vacuum pipe 48 inlet andthe chamber slit valve 11. When the heater plate is actuated to removethe substrate 24 from the chamber 10, the bypass line is opened tocommunicate between the grooves 77, 78 and the enclosure 13. Heaterplate top surface 26 may also include a groove, or plurality of grooves,located adjacent the outer circumference thereof which are not connectedto the vacuum. These grooves reduce the area of contact between thesubstrate 24 and heater plate 18, which reduces the heat transfer to,and thus deposited film thickness on, the substrate edge 27.

During sequential processing of substrates, it has been found thatsubstrates 24 may become sufficiently misaligned, with respect to thechamber components, that the substrate 24 may be tipped with respect toheater plate upper surface 26. Additionally, substrates 24 may becracked or warped. In each instance, the continued processing of thesubstrate 24 can allow the processing gas to contact the internal areasof the heater plate 18 which can effect the integrity of the heaterplate 18, can create particles, or can free portions of the crackedsubstrates into the chamber. In these instances, it is desirable toimmediately stop the processing to remove the substrate 24 beforechamber damage occurs. It has been found that in the situation where asubstrate 24 is misaligned, cracked or warped, the vacuum pressure atthe inlet to the vacuum pump, which maintains the vacuum pressure ingrooves 77, 78 in the heater plate 18, will change from that which ispresent when a flat, complete and properly positioned substrate is onthe heater plate 18. A pressure sensor 49 is located in the vacuum lineat the inlet to the vacuum pump to transmit a signal to a shut downcontroller, which shuts down chamber operation when the vacuum pressureis indicative of a cracked, warped or misaligned substrate. When asubstrate 24 is properly received on the heater plate 18, and theenclosure is maintained at about 80 torr, the pressure at the vacuumpump inlet, and thus at the sensor 49, will be 1 to 2 torr. Where asubstrate 24 which is substantially misaligned, cocked or substantiallywarped is received on the upper surface 26, the sensor 49 pressure willapproach less than 5 torr. With a cracked substrate, the pressure willrange from 10 torr up to the chamber pressure.

The Substrate Edge Protection System

Referring now to FIGS. 5 and 6, the preferred embodiment of thesubstrate edge protection system 30 is shown. When a substrate islocated on the upper surface 26 of heater plate 18, the substrate edgeprotection system 30 provides and a gas which passes about the perimeterof the substrate 24 to prevent material deposits on that area of thesubstrate 24. Substrate 24 has a circumferential edge 27 extendingaround the periphery thereof, which includes an upper tapered surface17, a lower tapered surface 19 and a generally flat middlecircumferential portion 21. To limit the occurrence of substrate 24defects caused by the dislodging of loose deposits on the edge 27 andunderside of the substrate 24, but simultaneously maximize the number ofuseful die produced from the substrate 24, the deposition layer shouldbe evenly deposited all the way to the edge 27 of the substrate, but notoccur on the underside, lower tapered surface 19 or flat middle portion21 of the substrate 24 where it may contact other materials and becomedislodged. The substrate edge protection system 30 of the presentinvention addresses this requirement.

Referring to FIG. 6, the upper surface 26 of heater plate 18 isconfigured to provide an integral purge gas channel 220 to supply arelatively constant supply of purge gas about the entire perimeter ofsubstrate 24. To provide purge gas channel 220, heater plate uppersurface 26 terminates in an upwardly projecting guide receiving portion222 which is an annular flat raised portion, disposed 0.002 to 0.005inches above upper surface 26. Purge gas channel 220 is formed as aninwardly extending groove, disposed at an approximately 135° angle fromupper surface 26, at the interface of upper surface 26 and the base ofthe inner edge of guide receiving portion 222. A plurality of purge gasholes 234 are disposed between the inner terminus of purge gas channel220 and the purge gas manifold 218, and are evenly spacedcircumferentially about the heater plate 18 to supply the purge gas frommanifold 218 into channel 220. The number of holes 234 is dependent uponthe intended distance between the substrate edge 27 and the bottom ofthe channel 220. With a distance from the point of entry of the hole 234into the channel 220 to the substrate edge 27 of 0.060 inches, thenumber of holes is approximately two hundred and forty. As the distancefrom the hole 234 opening to the wafer edge 27 increases, the number ofholes necessary to provide a constant flow of purge gas at substrateedge 27 decreases. Where the distance from the opening of hole 234 tothe substrate edge 27 is doubled, the number of holes 234 isapproximately halved.

Referring again to FIGS. 5 and 6, to precisely position the substrateedge 27 adjacent channel 220 for processing, the preferred substratealignment member 32 includes a plurality of ceramic guide pins 224disposed on guide receiving portion 222 adjacent channel 220. Each pin224 includes a front portion 226 tapered approximately 12 degrees fromvertical. Front portion 226 includes a generally flat extending centralportion 230, and tapered sides 228, such that central portion 230extends further inward toward the center of the heater plate uppersurface 26 than tapered sides 228. Central portion 230 extends inwardlyfrom guide receiving portion 222 and over purge gas channel 220. Eachpin 224 also includes a rearwardly extending mounting tab 231, whichincludes a pair of holes therein for receipt of bolts to secure the pin224 to guide receiving portion 222.

The guide pins 224 are located on heater plate 18 such that extendingcentral portions 230 of the pins 224 are disposed approximately five toseven thousandths of an inch from substrate flat middle circumferentialportion 21 when substrate 24 is perfectly aligned to the center ofheater plate 18. Thus, in the instance where the substrate 24 isperfectly aligned, the substrate 24 will come into contact with uppersurface 26 without touching any of pins 224. However, most substrateshave a slight amount of eccentricity, and the robot blade does notalways perfectly center the substrate 24 on upper surface 26. In thoseinstances, lower tapered surface 19 and flat middle circumferentialportion 21 of substrate 24 will engage one, or more, extending portions230 of guide pins 224, and the guide pins 224 will align the substrateinto position on upper surface 26 so that edge 27 does not substantiallyblock purge gas channel 220. By positioning the substrate 24 with guidepins 224, the only portion of the substrate 24 which touches thealignment mechanism is that small portion of edge 27 in contact withguide pin central portion 230. As central portion 230 extends radiallyinwardly from purge gas channel 220, substrate edge 27 will locate asmall distance from the channel 220, and the area of substrate 24 toeach side of the contact area of substrate 24 with central portion 230will receive an uninterrupted supply of purge gas. Although in thepreferred embodiment a purge gas is supplied to the substrate edge, thepresent invention specifically contemplates the use of reactive gasses,in addition to the purge gas, as the masking gas. By adding a reactivespecies, such as H₂ to the reactive gas, deposition at the substrateedge may be selectively enhanced if desired.

When the substrate 24 is first received on heater plate 18, thetemperature thereof may be substantially lower than the heater plate 18temperature. Once the substrate 24 comes into contact with the heaterplate 18, heat is transferred into the substrate 24 to raise itstemperature to the processing temperature. This temperature increasecauses thermal expansion of the substrate 24, such that edge 27 thereofmay press against alignment pins 224. Further, the vacuum in grooves 77,78 firmly adheres the substrate 24 to heater plate 18 upper surface 26,such that the edge 27 of substrate 24 may become compressively loadedagainst pin 224. As a result of this loading, the substrate 24 can crackor chip where it touches alignment pin 224. To minimize the occurrenceof chipping or cracking at the substrate edge 27, the chamber controllermay be programmed to maintain the chamber pressure in the vacuum grooves77, 78 during the period when the substrate 24 is heated, and then pulla vacuum through the grooves 77, 78 after the substrate 24 reaches astable temperature. The presence of chamber pressure below substrate 24allows the substrate to expand away from the area of contact withalignment pin 224, thus reducing localized compressive stresses andreducing the incidence of compressive cracking or chipping on thesubstrate edge 27. Additionally, purge gas may be backflushed throughvacuum grooves 77, 78 as the substrate 24 is received on upper surface26 to help align the substrate 24 and reduce frictional adhesion of thesubstrate 24 to support pins 25 as guide pins 224 guide the substrate 24into position, or gas may be backflushed through the grooves 77, 78 asthe substrate 24 thermally expands to allow the substrate 24 to expandaway from pins 224 when received on heater plate upper surface 26.

The Chamber Exhaust System

Referring again to FIGS. 2 and 3, the improved exhaust system 300 of thepresent invention is shown. The top 12 of chamber 10 includes a priorart manifold 23 which leads to the exhaust port 304 of the chamber 10.Suction through the exhaust port 304 pulls exhausted chamber gasses outof enclosure 13 to maintain the proper processing environment inenclosure 13. Manifold 23 extends substantially around the perimeter oftop 14, but a gap remains where wall 16 is pierced by the slit valve 11.This gap creates uneven exhausting and thus uneven chamber processinggas distribution throughout the enclosure 11. In accordance with theinvention, a pumping plate 308, with a plurality of apertures 29 evenlyspaced thereabout, is mounted over manifold 23. Apertures 29 are spacedapart approximately 30 degrees, and an aperture 29 is spaced at each endof manifold 23 adjacent the gap. The evenly spaced apertures in pumpingplate 308 create even exhausting of used chamber processing materials,which leads to the creation of a more uniform deposition layer on thesubstrate 24.

Conclusion

The foregoing embodiments of the invention provide a CVD chamber whichyields a more uniform deposition material layer on the substrate, whilesimultaneously reducing the incidence of particle generation duringprocessing. By eliminating the shadow ring which touches the substrateduring processing, and instead creating a uniform sheath of purge gasabout the substrate edge during processing, the overall yield of diefrom the substrates is increased by reducing localized temperaturevariation at the substrate edge and eliminating the masked edge of thesubstrate created by the shadow ring. Additionally, the uniformity ofthe deposition is enhanced, by creating a uniform exhausting of reactedproducts from the enclosure 13.

The structure of the improved CVD chamber also leads to reduced particlegeneration. Rubbing of the substrate 24 on heater plate 18 upper surface26 is reduced, by reducing the frictional adhesion of the substrate 24and heater plate 18 as the substrate is received thereon, by eliminatingcontact between the shadow ring and substrate 24, and by reducing theamount of deposition material received on the chamber components.

Although specific materials have been described for use with the presentinvention, those skilled in the art will recognize that the materials,and the arrangement of the components of the invention, may be changedwithout deviating from the scope of the invention. Additionally,although the invention has been described for use in a CVD chamber, thecomponents herein are equally suited for use in plasma deposition andother processes.

We claim:
 1. An apparatus for depositing a layer on a substrate,comprising:a chamber having an enclosure for receiving and processingthe substrate therein; a substrate support member disposed within saidenclosure; a stem interconnected to said substrate support member tomove said substrate support member within said enclosure; and a heatlimiting member disposed about a tubular portion of said stem to limitheat transfer along said stem.
 2. The apparatus of claim 1, wherein saidheat limiting member is a reduced area portion on said tubular portionof said stem.
 3. The apparatus of claim 1, further including a substrateedge protection member.
 4. The apparatus of claim 3, further including asubstrate alignment member to align a substrate on said substratesupport member with respect to said substrate edge protection member. 5.The apparatus of claim 1, further including;a shroud extending about theportion of said stem extending outwardly of said enclosure and formingan enclosed space about said stem; and a gas supply ported to said spaceto supply a protective gas about said stem.
 6. The apparatus of claim 5,wherein said space communicates with said enclosure.
 7. The apparatus ofclaim 1, further comprising a thermocouple disposed within a bore insaid substrate support member and maintained in communication with theconditions present on the exterior of said enclosure.
 8. The apparatusof claim 1, further comprising an exhaust manifold in communication withsaid enclosure, and an exhaust plate having a plurality of exhaustapertures therein provided intermediate said enclosure and said exhaustmanifold.
 9. The apparatus of claim 1, further comprising:a substrateedge protection member integrally formed in said substrate supportmember, said protection member including a gas manifold positioned todirect a purge gas along the edge of a substrate received on thesubstrate support member; an exhaust manifold in communication with saidenclosure; an exhaust plate having a plurality of exhaust aperturestherein provided intermediate said enclosure and said exhaust manifold;and a thermocouple disposed within a bore in said substrate supportmember and maintained in communication with the conditions present onthe exterior of said enclosure.
 10. The apparatus of claim 1, whereinthe stem is a tubular stem.
 11. An apparatus for depositing a layer on asubstrate, comprising:a chamber having an enclosure for receiving andprocessing the substrate therein; a substrate support member disposedwithin said enclosure; a stem interconnected to said substrate supportmember to position said substrate support member within said enclosure;and wherein said stem comprises a tubular portion which furthercomprises a region of reduced cross-sectional area to limit heattransfer along said tubular portion of said stem.
 12. The apparatus ofclaim 11, wherein said region of reduced cross-sectional area is aregion of reduced outside diameter on said tubular portion of said stem.13. The apparatus of claim 11, further including a substrate edgeprotection member.
 14. The apparatus of claim 13, further including asubstrate alignment member to align a substrate on said substratesupport member with respect to said substrate edge protection member.15. The apparatus of claim 11, further including;a shroud extendingabout the portion of said stem extending outwardly of said enclosure andforming an enclosed space about said stem; and a gas supply ported tosaid space to supply a protective gas about said stem.
 16. The apparatusof claim 15, wherein said space communicates with said enclosure. 17.The apparatus of claim 11, further comprising a thermocouple disposedwithin a bore in said substrate support member and maintained incommunication with the conditions present on the exterior of saidenclosure.
 18. The apparatus of claim 11, further comprising an exhaustmanifold in communication with said enclosure, and an exhaust platehaving a plurality of exhaust apertures therein provided intermediatesaid enclosure and said exhaust manifold.
 19. The apparatus of claim 11,further comprising:a substrate edge protection member integrally formedin said substrate support member, said protection member including a gasmanifold positioned to direct a purge gas along the edge of a substratereceived on the substrate support member; an exhaust manifold incommunication with said enclosure; an exhaust plate having a pluralityof exhaust apertures therein provided intermediate said enclosure andsaid exhaust manifold; and a thermocouple disposed within a bore in saidsubstrate support member and maintained in communication with theconditions present on the exterior of said enclosure.
 20. The apparatusof claim 11, wherein the stem is a tubular stem.